The present invention generally relates to the field of wear resistant coatings. In particular, the present invention relates to wear resistant coatings for carbon seals.
Successful operation and performance of gas turbine engine bearing compartment carbon seals is strongly dependent on having a hard, chemically stable, and thermal-shock resistant counterface material system. The most common arrangement involves a static carbon seal, spring and air loaded axially against a shaft co-rotating ring, known as a seal plate or seal seat. The counterface is defined as the region of the seal seat contacting the axial and/or radial face of the carbon seal.
Historically, the counterface material system has consisted of a low alloy steel protected with hard chromium plating (HCP) or by a chromium carbide-nickel chromium coating applied by a Detonation Gun (D-Gun), available from Praxair Surface Technologies, Inc. Seal applications using HCP are typically limited to lower speed applications, and the plating process generates a heavily regulated hexavalent-chromium waste stream. While a superior counterface to hard chromium plating, the chromium carbide-nickel chromium coating applied by the D-Gun can exhibit localized surface distress in the form of radial or craze-type cracks due to thermal-mechanical stresses during operation. The cracks occasionally propagate to the extent that the coating material is liberated from the coated surface, either as discrete pull-out or gross spallation.
Attempts have been made to either complement or improve upon the D-Gun technology by depositing coatings using the continuous combustion high velocity oxygen fuel (HVOF) method. These attempts have been generally unsuccessful for application to a seal seat coating running against gas turbine engine carbon seals. Potential reasons include: the coatings were developed for other types of wear applications involving different mating materials and operating environments; carbide type and chemistry not thermo-chemically stable for operation against carbon seals at high power; and microstructures, primarily phase morphology and size, were not optimized to resist the propagation of surface thermal cracks into the thickness of the coating, often resulting in a rapid and catastrophic breakdown of the coating and unacceptable levels of carbon seal wear. It would be beneficial to develop a coating applied by HVOF for use with carbon seals.